"What is the biological advantage, if there isn't some sort of feedback cycle with the successful genes returning south?"

Perhaps the moths have the same advantage as some fungal diseases that blow in fresh every year from the tropics to infect US Ag crops then die every winter.  Could simply be an organism that constantly attempts to expand its range by large distances moves easy when earth moves from ice ages to tropical climates and the reverse or when continental drift opens a new habitat while organisms that stay put die out?  Just being able to move could be the biological advantage.

" Any discovery made by the human mind can be explained in its essentials to the curious learner."  Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists.   "For every complex problem there is a solution which is simple, neat and wrong."  H. L. Mencken

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